Neuropeptides were studied as mediators of neuron-glial interactions that regulate the survival and differentiation of developing neurons. Primary cell culture preparations from the mammalian central nervous system were employed as targets for neuropeptides, sources of trophic agents, and test systems for neuronal survival and differentiation. Vasoactive intestinal peptide was shown to release a peptidergic growth factor from astroglial cells obtained from cerebral cortex of newborn rats. This growth factor was biochemically isolated and purified to homogeneity as determined by SDS polyacrylamide electrophoresis and isoelectric focusing. The purified material was shown to increase the survival of developing spinal cord neurons in electrically blocked cultures. The growth factor was designated Activity Dependent Neurotrophic Factor or ADNF, because of its relationship to ongoing spontaneous electrical activity. To further explore the biological activity of ADNF, neutralizing antisera was generated. The anti-ADNF was shown to produce neuronal cell death in developing spinal cord cultures. Vasoactive intestinal peptide was shown to produce a translocation of protein kinase C in astrocytes to the nuclear membrane. VIP plus phenylephrine treatment elicited calcium mobilization in astrocytes. Protease nexin I, a serine protease inhibitor released by VIP from astrocytes, was shown to increase the survival of developing neurons. Continued characterization of a novel VIP antagonist was performed in developing rodents where it produced increases in VIP binding in the thalamus and olfactory bulbs along with increased expression of VIP mRNA thalamus and cortex. ONB binding was not affected by the VIP antagonist. The regional development of VIP binding sites was characterized in the developing rat. NMDA was found to increase the expression of mRNA for methionine enkephalin in developing spinal cord cultures as determined by Northern blot analysis; whereas, agents which stimulate non-NMDA glutamate receptors produced a decrease in methionine enkephalin mRNA.